Maximizing the production of lumber from each log is a major objective of sawmills of today. Many different types of machinery or apparatus are applied to this task. An example is the chip-and-saw apparatus. A log is scanned and a computer determines a precise rectangular cross section extended lengthwise down the log that can be derived from the log and the precise cuts that can be made to maximize the production of lumber from that cross section. The log is then passed through a series of chippers that discriminately removes the wood of the log periphery to generate the desired cross section. Included is a bottom chipper that squares (or flattens) the log bottom, side chippers that square the log sides, and a top chipper that squares the log top, all precisely in accordance with the dictated rectangular cross section determined by the computer.
The chippers have to be adjusted relative to each log to be sawn in order to accomplish the desired flattening (or opening) of the sides. Typically the bottom chipper is fixed and the log infeed or conveyor path, as defined by the height of the infeed table is raised or lowered relative to the bottom chipper. The side chippers are moved in and out relative to a center line and the log infeed path is laterally adjusted to align the log with that center line. The top chipper is raised and lowered as needed.
The mechanism to which the present invention is directed is that mechanism which raises and lowers the infeed table or conveyor. Logs being arranged on the table can weigh several thousand pounds and the weight of the conveyor mechanism which supports the log can weigh additional thousands of pounds. The mechanism which adjusts the height of this very large weight must be rapid and precise. The log conveyance path defined by the table cannot be altered except in height and side way shifting, i.e., the entire table must be equally raised so as to be retained in a parallel plane.
The mechanism to achieve this task has progressed through several stages. In an early version of the table lift mechanism, the conveyor structure is supported on a series of lateral shafts hereafter referred to as lifting shafts. The lifting shafts were supported at each end on a pivotal crank arm, the connection to the crank arm being offset a precise distance from the crank arm's pivotal axis. Simultaneous pivoting of all of the crank arms simultaneously raise or lower the lifting shafts which in turn raise or lower the table, with every position of the table retained in a parallel plane.
As previously mentioned, each lifting shaft was supported on a set of crank arms. The crank arms of each set were connected through their pivotal axis by a pivoting shaft. Pivoting one crank arm of a set would apply torque to the pivot shaft resulting in rotation of the pivot shaft to pivot the other pivot arm of the set.
To insure simultaneous and equal movement of all the crank arm sets supporting the lifting shafts, a single actuator was utilized. A cylinder was coupled to one crank arm of a set. A rigid rod coupled the cylinder actuated crank arm to one crank arm of the next crank arm set and the additional crank arm sets would be coupled to the previous crank arm set by additional rods in the same manner.
The cylinder engaging one of the opposed crank arms connected together by the pivoting shaft, is controlled by the computer to actuate pivotal movement of the crank arms and corresponding raising and lowering of the infeed table.
The problem with the above-described mechanism is that the single actuator applies excessive torque to the pivotal shaft. Failure of the pivotal shaft and/or various connecting means used for connecting the crank arm and pivoting shaft are common.
A second version was developed whereby the pivotal shaft was eliminated and an actuator was applied to a crank arm on each side of the table. The problem with this design was the precise timing required of the actuators. The slightest difference in actuation is intolerable, as everything is tied together by support beams and the huge forces applied to the crank arms at each side causes severe damage when not equally applied.